How much contingency, and on what base?

Your financier expects a contingency line at an agreed percentage, calculated on the right amount. In Splinde it is a percentage you choose, built either as a Markup AddOn on a Cost Item or as its own Cost Item whose Fee points with @ at a Section total. For the whole budget, wrap everything in one top-level Section and point at that Section.

The rate is a decision between you and the financier. Splinde's own guidance gives 5% for a low-risk job, 10% as the standard (a hard floor), 15% for high risk and 20% for an extreme shoot, with a completion bond fee of 2 to 3% of the total budget sitting on top of that as a separate cost. Confirm the number against the agreement in front of you. Two errors are worth avoiding: treating a completion bond as if it were contingency, and calculating contingency before fringes are added, which undercounts it.

Contingency on top of fees and fringes

AddOns do not compound by default. Every percentage AddOn on a line takes the same starting amount, the line's own Amount, days and Fee. To put contingency on top of a fringe:

Drag the Markup card from Insert, AddOns, onto the Cost Item and expand it.
Open the selection menu in the AddOn's sentence line. It lists the line's Base Sum and every other AddOn on the line, each with a checkbox.
Tick the fringe AddOn as well as the Base Sum. The contingency now calculates on fee plus fringes.
Type the rate in the rate field, or type @ and pick a Variable of type % Percentage, so one edit changes it everywhere.

Contingency on a Section total

Create a Section for the contingency and drag a General Cost Item into it.
In Fee, type @, open the Sections tab and pick the Section you want to calculate on.
Click the unit label above the middle number field, search for Percentage in Search Unit, and pick it. Then type the rate there, or type @ and pick a % Percentage Variable. Set the unit first, so the figure you type is the percentage itself.
Leave Amount at 1.

Contingency on the whole budget

Create one new top-level Section, for example Production costs, and drag your other Sections onto it so they all sit inside.
Put the contingency line outside that Section, beside it, so it adds to the total instead of counting itself.
In the contingency line's Fee, type @, open the Sections tab and pick Production costs.
Set the Percentage unit and the rate as above.

Give the completion bond its own Cost Item the same way, with its own rate Variable, so the financier reads contingency and bond as two separate lines.

Good to know

The @ menu offers Variables, Cost Items and Sections, which is why a Section that holds your whole budget gives the contingency line one place to point.